Oxygen free radicals react with DNA to cause strand breaks and formation of base adducts. Recently, attention was focused on one specific product, 8-hydroxyguanine, formed by the reaction of hydroxyl free radicals with DNA. This altered base appear to be involved in miscoding of DNA readout and; therefore, could be involved in the toxic and mutagenic action of oxygen free radicals. We have discovered that treatment of DNA with methylene blue (MB) plus light causes efficient formation of 8 hydroxyguanine with apparently very little other damage to DNA. It also appears that treatment of RNA with MB plus light also causes formation of 8-hydroxyguanine in this nucleic acid. Evidence is presented to indicate that MB plus light causes 8-OHG formation in nucleic acids by a single oxygen mediated mechanism. We propose to investigate the mechanism involved in the MB plus light mediated formation of 8-OHG in DNA and RNA. We have also found that MB plus light causes inactivation of R17, and RNA virus in a very short time with low concentrations of MB in the presence of light. We propose to investigate the mechanisms involved in R17 inactivation. Also we have found that MB plus light causes mutations in DNA as assessed by the M13 mp2 plasmid system. We propose to investigate, the spectrum of mutations produced by MB plus light.